IndexedDB: delete LevelDBScopesFactory.

This interface is used in exactly one place, has exactly one
implementation, exactly one method, and zero state. Thus, inline it.

This is part of an ongoing effort to reorganize IDB to:
* move each backing store into its own thread
* make it possible to replace LevelDB with SQLite

Simplifying initialization code is a precursor to moving it to where
it really belongs, which is not in IndexedDBFactory (which should be
backing store-agnostic) and is *probably* not on the main IDB sequence.

Bug: 1474996
Change-Id: I9cba18bcb538e2f947dc154a461f798084232918
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4998100
Reviewed-by: Ayu Ishii <ayui@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1218464}